Variable resistance materials are promising active materials for next-generation electronic storage and computing devices. A variable resistance material is a material that possesses two or more states that differ in electrical resistance. The material can be programmed back and forth between the states by providing energy to induce an internal chemical, electronic, or physical transformation of the material that manifests itself as a change in resistance of the material. The different resistance states are distinguishable and can be used as memory states to store or process data.
Phase change materials are a promising class of variable resistance materials. A phase change material is a material that is capable of undergoing a transformation, preferably reversible, between two or more distinct structural states. The distinct structural states may be distinguished on the basis of, for example, crystal structure, atomic arrangement, order or disorder, fractional crystallinity, relative proportions of two or more different structural states, or a physical (e.g. electrical, optical, magnetic, mechanical) or chemical property. In a common embodiment, the two or more distinct structural states include differing proportions of crystalline phase regions and amorphous phase regions of the phase change material, where the phase-change material is reversibly transformable between the different states. In the crystalline state, the phase change material has lower resistivity; while in the amorphous state, it has higher resistivity. Continuous variations in resistivity over a wide range can be achieved through control of the relative proportions of crystalline phase regions and amorphous phase regions in a volume of phase-change material. Reversibility of the transformations between structural states permits reuse of the material over multiple cycles of operation.
Typically, a programmable resistance device is fabricated by placing the active variable resistance material, such as a phase change material, between two electrodes. Operation of the device is effected by providing an electrical signal between the two electrodes and across the active material. In a common application, phase-change materials may be used as the active material of a memory device, where distinct data values are associated with the different structural states and each data value corresponds to a distinct resistance range of the phase-change material. The different structural states employed in memory operation may also be referred to herein as memory states or resistance states of the phase-change material. Write operations in a phase-change memory device, which may also be referred to herein as programming operations, apply electric pulses to the phase-change material to alter its structural state to a state having the resistance associated with the intended data value. Read operations are performed by providing current or voltage signals across the two electrodes to measure the resistance. The energy of the read signal is sufficiently low to prevent disturbance of the structural state of the phase-change material.
Phase-change memory devices are today typically operated in binary mode. In binary mode, the memory is operated between two structural states. To improve read margin and minimize read error, the two structural states for binary operation are selected to provide a large resistance contrast. The range of resistance values of a phase-change material is bounded by a set state having a set resistance and a reset state having a reset resistance. The set state is a low resistance structural state whose electrical properties are primarily controlled by the crystalline portion of the phase-change material and the reset state is a high resistance structural state whose electrical properties are primarily controlled by the amorphous portion of the phase-change material. The set state and reset state are commonly employed in binary operation and may be associated with the conventional binary “0” and “1” states.
In order to expand the commercial opportunities for phase-change memory, it is desirable to identify new phase-change compositions, device structures, and methods of programming that lead to improved performance. A key performance metric for memory devices is storage density, which is a measure of the amount of information that can be stored per unit area of memory material. Miniaturization is the most common strategy for increasing storage density. By shrinking the area required to store a bit of information, more bits can be stored in a memory chip of a given size. Miniaturization has been a successful strategy for increasing storage density over the past four decades, but is becoming increasingly more difficult to employ as fundamental size limits of manufacturability are reached.
An alternative approach for increasing storage density is to increase the number of bits stored in a given area of memory. Instead of reducing the area in which information is stored, the objective is to store more bits of information in a particular area of memory. In conventional binary operation, only a single bit of information is stored in each memory location. Higher storage density can be achieved by increasing the storage capacity of each memory location. If two bits, for example, can be stored at each memory location, the storage capacity doubles without miniaturizing the memory location. In order to increase the storage capacity of each memory location, it is necessary for the memory material to be operable over more than the two states used in binary (single bit) operation. Two-bit operation, for example, requires a material that is operable over four distinguishable memory states.
Phase-change memory materials have the potential to provide multiple bit operation because of the wide resistance range that separates the set and reset states. In a typical phase-change memory device, the resistance of the set state is on the order of ˜1-10 kΩ, while the resistance of the reset state is on the order of ˜100-1000 kΩ. Since the structural states of a phase-change material are essentially continuously variable over the range of proportions of crystalline and amorphous phase volume fractions extending from the set state to the reset state, memory operation of a phase-change material at memory states having resistances intermediate between the set resistance and reset resistance is possible. As a result, multiple bit memory operation over multiple memory states is achievable with phase-change memory materials.
Although phase-change memory offers the potential for multiple bit operation, progress toward achieving a practical multilevel phase-change memory has been limited. One of the practical complications associated with multilevel phase-change operation is resistance drift over time. It is common in phase-change memory devices to observe a variation in the resistance of a memory state over time. If a phase-change memory device is programmed into a particular state having a particular resistance at one time, the resistance of the device at a later time may be different. As a general rule, resistance increases with time and becomes more pronounced as the amorphous phase volume fraction of a structural state increases. Resistance drift is not problematic for binary operation of phase-change memory because the set state shows little or no drift in resistance over time, while the reset state shows an increase in resistance over time. As a result, the resistance contrast between the set and reset states increases over time and no impairment of performance occurs.
Resistance drift, however, becomes problematic in multilevel applications of phase-change memory devices because time variations in resistance may lead to overlap in the identification of memory states. In order to advance the performance capabilities and commercial potential of phase-change memory, it is desirable to develop phase-change materials, device structures or methods of operating phase-change memory devices that eliminate, counteract, or circumvent resistance drift.